World No 1 Aryna Sabalenka turned in an agitated display as she was beaten by Coco Gauff in a compelling WTA Finals last four clash in Riyadh.
Gauff reached her first final at the WTA Finals as she overcame a rival who produced a string of error at crucial points, handing the American a chance to seat a 7-6(4), 6-3 win
Gauff, aiming to become the first American champion since Serena Williams 10 years ago, will face Chinese debutante Zheng Qinwen in Saturday's final.
Sabalenka made an error-strewn start to the match as she coughed up error after error on her usually formidable forehand, yet she broke to lead the opening set 6-5, only for another forehand miss to let Gauff take it to a tie-break.
As Sabalenka again misfired badly, Gauff just had to hold herself together to move a set ahead.
A despondent Sabalenka trudged back to her seat after dumping a backhand into the net to let Gauff break for 2-1 in the second.
Another break, to love, gave Gauff four games in a row but Sabalenka pulled one back in a marathon sixth game, squandering seven break points before finally converting the eighth.
The jitters immediately resurfaced, though, as Sabalenka was broken to love again.
Yet again she hit back, but when Gauff forced a first match point on the Sabalenka serve, the Belarussian hit the net and the young American was through.
"I'm happy with the way I played," Gauff said in her on-court interview. "I know Aryna is always going to be a tough match so I just tried to stay in it. She's world number one for a reason."
Gauff beat Zheng in their only previous match, on clay in Rome earlier this year.
"She's playing great tennis," added Gauff. "I'm not really nervous. The year-end is just a plus and a reward for the season that I had.
"Yesterday I lost and I was able to brush that off and come in today and play a world No 1. She was the favourite and I am happy I was able to get through.
"I was getting broken and breaking back so I was positive for the most part. Being down a break, I just know I have to play for every point."
Zheng went into her semi-final against Barbora Krejcikova having won 30 of her last 35 matches and powered her way to a 6-3, 7-5 victory.
She is the second Asian player to reach the final of the season-ending event after her compatriot Li Na.
Wimbledon champion Krejcikova had beaten Gauff on Thursday to book her place in the last four and eliminate defending champion Iga Swiatek.
Zheng looked in complete control at a set and 3-0 up but one poor service game let Krejcikova back in and the Czech almost forced a decider.
However, 22-year-old Zheng forced a break of serve in the 11th game of the second set and went on to take her second match point.
